Future wire-free data transmission for high data transmission rates will occupy a very wide bandwidth. Standards for UWB (Ultra Wide Band) applications provide a frequency range of 3 to 10 GHz. In order to transmit data over this bandwidth range, a large number of transmission bands, for example for OFDM (Orthogonal Frequency Division Multiplexing) or DS-CDMA (Direct Sequence Code Division Multiplexes), must be generated in this frequency range.
Frequency pattern generators or hopping frequency synthesizers are generally used for this purpose. Frequency pattern generators of this type must provide frequency pulses having an adjustable time duration and a frequency that varies from pulse to pulse. Such pulsed output signals then form a suitable frequency spectrum that has a large number of transmission bands over an extremely wide frequency band range (UWB=Ultra Wide Band).
FIG. 1 shows one possible sequence of frequency pulses in the time domain. An exemplary pulsed signal RFOUT is illustrated as a function of time t. A first frequency pulse of duration Tp at the frequency f1 is followed, after a period of time Tb, by a second frequency pulse at a second frequency f2. A third frequency pulse at a third frequency f3 is also shown by way of example.
Ideally, no signal occurs between the frequency pulses. Depending on the UWB application, the pause between frequency pulses may be very short, down to Tb=Tp. The period of time between two pulses at different frequencies is generally in the nanosecond range.
A multiband generator for generating frequency pulses was proposed in the document IEEE 802.15-03/207r0, G. Shore et al. “TG3a-Wisair contribution on multi band implementation”, http://grouper.ieee.org/groups/802/15, May 2003.
FIG. 2 schematically illustrates a corresponding multiband generator MBG. In the latter, a clock signal CLK at 5280 MHz is supplied to a subband generator SBG. The subband generator SBG has a number of frequency dividers (not illustrated here) and outputs reduced frequency signals LF1, LF2, LF3, LF4 at the frequencies 440, 880, 1320, 1760 MHz. The subband generator also passes on the clock signal CLK as a carrier frequency signal LO.
A quintuple multiplexer MUX is provided and outputs either the reduced frequency signals LF1, LF2, LF3, LF4 or a signal DC at a constant level to a single-sideband mixer SSBU/L as a mixing frequency signal LF. The single-sideband mixer SSBU/L also receives the radio frequency clock signal as a local oscillator signal LO.
The single-sideband mixer SSBU/L thus mixes the fixed carrier frequency signal LO with the switched mixing frequency signals LF1, LF2, LF3, LF4 to form an output signal MBOUT of the multiband generator MBG. Different frequencies of the output signal MBOUT are thus achieved by switching the mixing frequencies LF1, LF2, LF3, LF4. Frequency pulses are generated by routing the signal DC at a constant level to the single-sideband mixer SSBU/L as a mixing frequency signal between switching from one mixing frequency, for example LF1, to a second mixing frequency, for example LF2.
A multiband generator in accordance with the prior art, as is shown in FIG. 2, has a number of disadvantages. Since the mixing frequency signals LF1, LF2, LF3, LF4 are generated from a clock signal CLK, higher harmonic components which have to be removed downstream of the multiplexer by means of analog low-pass filters can easily be generated in the subband generator SBG. Low-pass filters of this type in the path for the mixing frequency signals LF disadvantageously slow down the switching properties for switching between various frequencies.
A further disadvantage resides in the fact that the radio frequency carrier frequency LO scatters into the output signal MBOUT even when a constant level is applied, as the mixing frequency signal LF, to the single-sideband mixer SSBU/L. No clean frequency pulses and, in particular, no well-defined pauses between the frequency pulses are thus produced.